1. Field of the Invention
The present invention relates to a field-emission cold cathode and its method of manufacture, and particularly to field-emission cold cathode having a current-control element connected to emitters, and a method of manufacturing such a field-emission cold cathode.
2. Description of the Related Art
A field-emission cold cathode is an element having acute cone-shaped emitters and a gate electrode formed in proximity to the emitters and provided with openings on the submicron order, whereby the field-emission cold cathode focuses a high electric field at the tips of the emitters and emits electrons from the emitter tips in a vacuum. However, because emitters and gate in such an element are in extremely close proximity, a discharge may occur under the influence of, for example, residual gas during operation. As a result, a high current may flow to an emitter, causing melting of the emitter and short-circuiting between the emitter and gate, and as a consequence, breakdown of the element.
As a countermeasure to such problems, cold cathode elements have been developed in which a resistance layer is formed in series with the emitters so as to prevent melting and damage to emitters by controlling the current during a discharge. As one method of forming such a resistance layer, a simple method has been proposed in which a high-resistance region is formed in one portion of the cone-shaped emitter. However, because a potential difference of, for example, 100 V or more is applied between the emitter and gate, this construction results in the build-up of a voltage of 100 V or more across the high-resistance layer when a discharge causes a short-circuit to occur between the emitter and gate. In such a case, a high electric field produced in the high-resistance region causes field breakdown of the silicon film which forms the high-resistance region, or causes an avalanche effect in the high-resistance region, thereby producing flow of a large current and thus giving rise to a possible melt-breakdown of the emitter or gate. Constructions have therefore been reported in which the resistance length is lengthened such that a high field may not build up in the resistances layer. As an example, U.S. Pat. No. 5,475,280 discloses a method of lengthening the emitter cones.
The construction of such a field-emission cold cathode is shown in FIG. 1. Referring to FIG. 1, acute emitter 9 is formed in a pen shape on silicon substrate 1 with an interposed resistance layer 11, insulation layer 61 and gate electrode film 7 being formed so as to enclose emitter 9 and resistance layer 11. Forming the lower region under cone-shaped emitter 9 in a long pen shape and forming resistance layer 11 in this region enables to determine the effective length of resistance at substantially desired value. Thus, the resistance length can be set such that the voltage applied across the resistance layer may not exceed the breakdown field intensity or avalanche field intensity of silicon even in case that a high voltage builds up at the emitter tip and a consequent rise of a voltage takes place between both ends of the resistance layer when discharge or the like occurs.
This prior-art example is a method of forming resistance in the lower portion of an emitter, but other methods exist in which a resistance layer is formed separate from the emitter. FIG. 2 shows an example of the preceding technology (the technology preceding the present invention) in which emitters 9 are formed on silicon substrate 1 with insulator film 6 and gate electrode film 7 enclosing emitters 9. Trenches 3 are formed between emitters 9 to separate silicon substrate 1 into regions directly underlying each emitter. Trenches 3 are buried with insulative buried film 4. FIG. 3 shows a plan view of the cold cathode element of FIG. 2. As shown in the figure, by separating silicon substrate 1 into regions directly underlying each of emitters 9 using trenches 3, the regions of silicon substrate 1 surrounded by trenches 3 function as resistance layers connected to the corresponding emitters 9. By shortening the distance between emitters 9 and trenches 3 to form trenches at positions such that current flowing from emitters 9 may not spread, the resistance value of resistance regions enclosed by the trenches can be made uniform in the direction of thickness (depth)
This allows preventing local field to concentrate, and thereby an element with a high withstand-voltage resistance to be produced.
As explained hereinabove, forming resistance layers, having a width such that current does not spread, in series to each emitter and setting the resistance length such that field breakdown or avalanche breakdown does not occur enables both limit on the value of the current that will flow through the emitters as well as prevention of breakdown of the emitter caused by discharge. As an example, when a gate voltage is 100 V, the depth of the trenches (resistance length) must be at least 10 .mu.m to limit field intensity to 10.sup.5 V/cm or less to prevent avalanche breakdown.
A first drawback of the field-emission cold cathode of the above-described prior art is that formation of a long resistance layer in the lower portion of each emitter 9 as described in the first prior-art example results in a higher aspect ratio of emitters, and this makes downsizing of the element difficult. In other words, reducing the size of the emitters while maintaining a fixed aspect ratio means reducing the diameter of the emitters. On the other hand, keeping the diameter of the emitters at a constant while increasing the resistance length to 10 .mu.m or more to increase withstand-voltage inevitably increases the aspect ratio of the emitters. Either case causes a reduction in yield due to, for example, broken emitters when forming the emitters, and imposes a limit on miniaturization of the element. The same drawback also applies in the above-described preceding technology when the distance margin between the trenches and each emitter is reduced to bring about miniaturization. Moreover, since the width of the resistance regions underlying emitters (substrate portions demarcated by trenches 3) is decreased, an increase in resistance as well as a build-up of voltage during normal operation is caused.
As for a second drawback, in a method in which emitters are formed on a substrate surrounded by trenches as in the example of the preceding technology above, increase in the distance between emitters and trenches results in effectively all voltage applied across the base and emitter being placed locally in the contact region between the emitter and the substrate that directly underlies emitters when discharge occurs. This brings about field breakdown or avalanche breakdown. The reason for this local application of voltage is that a great distance between emitters and trenches causes current flowing from an emitter to spread radially outwards from the contact portion. The resistance for this radially spreading current is extremely high directly below an emitter (in the vicinity of the contact portion) and falls precipitously with increasing distance from the contact portion. As a result, essentially all of the voltage impressed between base and emitter is impressed to the region directly underlying this emitter when discharge occurs between base and emitter. In the following description, current spreading outwards from the emitter is referred to as spread current, and resistance that acts against this spread current is referred to as spread resistance.
Here, if the specific resistance of silicon substrate 1 is .rho., and if, for the sake of simplifying the discussion, the contact surface between the emitter and substrate is assumed to be a point p, then the current spreads out spherically within substrate 1 with point p as the current source, provided that .rho.=constant. Accordingly, the resistance of the hemispherical shell demarcated by the concentric spheres of radius r and radius (r+.delta.r) that take point p as center, i.e., the spread resistance .delta.R(r) at radius r is: EQU .delta.R(r)=.rho...delta.r/(2.pi.r.sup.2) (1)
As a result, the total spread resistance of the current flowing from one emitter is obtained by integrating equation (1) with respect to r for the position of-trench 3 taken as a boundary condition. As can be seen from equation (1), the spread resistance is inversely proportional to the square of radius r, and spread resistance is therefore extremely high compared to other regions in the case that radius r is small, i.e., directly below an emitter. As a result, the spread resistance directly below an emitter effectively accounts for all spread resistance.
If the distance between emitters and trenches is increased to avoid discharge, the surface of a substrate region surrounded by trenches unavoidably enlarges beyond the emitter base surface. In such a case, the current which flows from the emitter radially spreads so far as the trenches. The current, after arriving at the trenches, flows in the direction of the depth of the substrate 1 as a linear flow.
Accordingly, the current experiences two kinds of resistance, i.e., spread resistance and linear resistance.
Of these, the spread resistance which associates with the substrate region directly below emitters is short in length, while major part of the voltage applied between the emitter and gate is placed in the spread resistance when discharge takes place. Consequently, the electric field of the region across which current spreads is relatively high. As a result, the spread resistance directly below emitters effectively becomes responsible for the principal cause of field breakdown, thereby raising the possibility of problems of reliability such as deterioration of withstand-voltage.
Equation (1) is based on the very rough approximation that the contact surface between an emitter and silicon substrate 1 is regarded as a point, and a quantitative inference therefore cannot be drawn based on this equation. However, actually measured values are as follows:
In a case in which an emitter is formed with a distance margin of 1 .mu.m from trenches on a silicon substrate having a specific resistance of, for example, 5 .OMEGA.cm, the resistance of a region of the spread current exhibits a saturation tendency after spreading across a distance of approximately 0.2 .mu.m to 0.5 .mu.m, the resistance here being on the order of 50 k.OMEGA.. When a voltage of 100 V is applied between the gate and emitter in a case in which the resistance of a substrate region surrounded by trenches 10 .mu.m in depth is 50 k.OMEGA., the voltage applied to the resistance in the interior of the trenches (this resistance refers to the resistance of the region of the substrate surrounded by trenches. After spreading from an emitter as far as the trenches, current flows inwards of the substrate in the direction of depth of the trenches. The resistance in the interior of trenches is therefore not spread resistance but resistance to normal linear current.) is 50 V, and the voltage applied across the region through which current spreads is also 50 V. The field intensity at this time is 5.times.10.sup.4 V/cm because a voltage of 50 V is impressed across a distance of 10 .mu.m in the resistance interior of the trenches. However, the field intensity in the spread-resistance region becomes 5.times.10.sup.5 V/cm because a voltage of 50 V is applied across a distance of approximately 1 .mu.m in a radial direction in this region. As a result, field breakdown is possibly caused in the spread-resistance region. The possibility of field breakdown in the region of current spread, i.e., in the spread-resistance region, thus increases in cases in which the distance margin between trenches and emitters is made greater than the 0.2 .mu.m to 0.5 .mu.m length.
Furthermore, a third drawback is that a method in which a trenches are formed for each individual emitter imposes a limit upon the reduction of space between emitters, i.e., limits miniaturization. In other words, in cases in which a trench depth of 10 .mu.m or more is required, attempting to produce an emitter pitch on the order of 2 .mu.m or less causes the trench width to fall below 1 .mu.m and the aspect ratio of trenches to exceed 10. This causes difficulties when burying trenches with a buried layer.
Here, a method is considered in which the distance between emitters is reduced by forming a plurality of emitters in an area surrounded by trenches. However, mere forming a plurality of emitters within an area surrounded by trenches results in greater current spread from each individual emitter than for a case in which each individual emitter is separated by trenches. This results in the problem that high withstand-voltage cannot be obtained because, as explained hereinabove, withstand-voltage is essentially determined by the conditions in the region directly below an emitter.